A force sensor, which has a function of outputting a force applied in a predetermined axis direction and a torque applied around a predetermined rotation axis as electric signals, is disclosed in Patent Literature 1, for example, and is widely used for force control of industrial robots. In recent years, such a force sensor has been also employed in life support robots, and there is a demand for high safety. However, a current capacitance type force sensor is provided with a mechanism portion, a capacitance detection portion (force detection portion), and an electronic circuit including a microcomputer, for example, and is likely to malfunction due to condensation, an impact, an overload, or mixing a foreign substance between a pair of parallel flat plates which provide the capacitance. In particular, a force detection portion of the force sensor has flexibility, and thus, metal fatigue is caused by the overload or a repeated load. As a result, a crack or the like may be generated in an elastic body forming the force detection portion, and there is a risk that the elastic body may be eventually broken.
As a simple method of determining whether a force sensor malfunctions, a plurality of the force sensors (for example, three force sensors) described in Cited Document 1, for example, may be arranged in parallel to evaluate a difference between output signals of the respective force sensors. In this method, three output signals are compared two by two, and when a difference between output signals of two force sensors falls within a predetermined range, it is determined that the force sensors normally function. On the other hand, when the difference does not fall within the predetermined range, it is determined that the force sensors do not function normally (malfunction).